1. Field of the Invention
The present invention relates to a call admission control device and call admission control method, and more specifically to a call admission control device and call admission control method for controlling the admission of a call in a packet communication system in mobile communications.
2. Description of the Related Art
A mobile communication system performs a communication using finite resources (frequencies and power). There is an upper limit to the communication capacity. Therefore, it is necessary to limit the number of mobile stations in a cell depending on the communication capacity. Practically, when a new mobile station starts communications in the cell, it is necessary to judge whether or not the new mobile station can start a communication in the cell. The control is referred to as call admission control. The status in which a new mobile station cannot start a communication in the cell, that is, the status in which the communication capacity is being used substantially 100%, is referred to as a capacity limit.
The conventional call admission control is a control method of, for example, allowing a new mobile station to start communications when the total number of mobile stations being communicating in the cell does not exceed a predetermined threshold value, and not allowing a new mobile station to start communications when the total number of mobile stations being communicating in the cell exceeds the predetermined threshold value.
Relating to the standardization of a third generation mobile communication system, what is called IMT (International Mobile Telecommunications)-2000, the standard specifications related to the W-CDMA-system and the cdma 2000 system are prescribed respectively for the 3GPP (Third-Generation Partnership Project) and 3GPP2 (Third-Generation Partnership Project 2).
In the 3GPP, with the recent fast spread of the Internet, based on the prediction that high speed and large capacitance traffic by the download, etc. from a database and a Web site especially in the downlink, the specification of the “HSDPA (High Speed Downlink Packet Access)” as a high speed packet transmission system in the downlink direction is prescribed (for example, refer to “3GPP TR25.848 v4.0.0”). As for the 3GPP2, from a similar point of view, the specification of the “1×EV-DO” as a high speed dedicated transmission system in the downlink direction is prescribed (for example, refer to “3GPP2 C.S0024 Rev.1.0.0”). In the CDMA 2000 1×EV-DO, the DO means “Data only”.
Further described below is the HSDPA.
The HSDPA is a system for performing communications by sharing one shared channel or more than two channels among a plurality of mobile stations, and a radio base station selects a mobile station which uses the shared channel from among the plurality of mobile stations for each TTI (Time Transmission Interval, 2 ms for the HSDPA) for transmission of a packet. In this case, depending on the occurrence of data traffic, there is a difference in number of mobile stations in a cell when a capacity limit is reached. For example, when the case where all mobile stations download data using an FTP (File Transfer Protocol) is compared with the case where all mobile stations perform Web browsing, the number of mobile stations in the cell when the capacity limit is reached is larger in the latter case because there are all the time packets to be transmitted to the mobile stations in downloading data using the FTP, while, in the case of the Web browsing, a user has reading time to browse Web pages and there is time when no packet to be transmitted to a mobile station exists. That is, in the case of the Web browsing, since there is a low frequency at which one mobile station uses a shared channel, more mobile stations can share one shared channel. As a result, in the HSDPA, the number of mobile stations that can be accumulated in a capacity limit fluctuates depending on the occurrence of data traffic.
In the HSDPA, a modulation system of a radio channel and a system of controlling a coding rate (in the HSDPA, it is called an AMCS (adaptive modulation and coding scheme)) are adopted depending on the radio channel quality between a mobile station and a radio base station, and the transmission data rate fluctuates depending on the radio channel quality (for example, signal-to-interference power ratio (SIR)) between the mobile station and the radio base station. On the other hand, the radio channel quality (SIR) largely depends on the mode of a cell such as an outdoor environment and an indoor environment, an urban area and a suburb, etc. That is, in the HSDPA, the number of mobile stations that can be accumulated at the capacity limit fluctuates depending also on the mode of a cell.
In such HSDPA, when the call admission control in the above-mentioned conventional method, that is, the call admission control based on the number of mobile stations currently performing communications in a cell, is performed, the number of mobile stations performing communications in the cell in a capacity limit is constant. Therefore, the transmission data rate of each mobile station in a capacity limit depends on the position of each mobile station or the mode in a cell. For example, since there is little interference from another cell in an indoor environment, and the radio channel quality is high, the transmission data rate of each mobile station in a capacity limit is high. However, since there is much interference in an outdoor environment, and the radio channel quality is not high, it is considered that the transmission data rate of each mobile station in a capacity limit is low.
However, the transmission data rate of each mobile station in a capacity limit is to be determined by the serviceability of the communications provided using the HSDPA, and it is desired that the speed is constant regardless of the mode of a cell or the position of each mobile station.
Described below is the scheduling in the HSDPA. As described above, the HSDPA is a system for performing communications by sharing one channel or more than two channels to be shared among a plurality of mobile stations, a radio base station selects a mobile station that uses the shared channel from among the plurality of mobile stations for each TTI (Time Transmission Interval, 2 ms in the HSDPA), and transmits a packet. Selecting a mobile station that uses the shared channel for each TTI by a radio base station is referred to as scheduling.
An well known algorithm of the scheduling is a round robin scheduler for controlling the transmission order of a transmission allocation packet of a downlink shared channel in order (for example, the mobile station #1→#2→#3→ . . . ) on the mobile station which are connected to a radio base station device. Furthermore, a Proportional Fairness scheduler and a MAX C/I (Maximum C/I) scheduler for controlling the transmission order of a packet waiting for transmission based on a radio channel quality of each mobile station and an average transmission data rate of each mobile station are also well known.
For example, the Proportional Fairness scheduler performs scheduling by calculating the evaluation function Cn=Rn/avrgRn relating to each mobile station, and allocating a packet to a mobile station having the largest evaluation function Cn. Rn indicates a momentary radio channel quality of each mobile station, and avrgRn indicates an average transmission data rate of each mobile station.
The Proportional Fairness scheduler can be expected to obtain higher throughput than the round robin scheduler because transmission allocation is performed in a state in which downlink radio channel quality is relatively good in each mobile station. Then, the value of the evaluation function expression of a mobile station having a high average transmission data rate is reduced by performing a division by an average transmission data rate of each mobile station, thereby realizing allocation with high fairness in time.
Furthermore, from a view point of serviceability, a method of providing Proportional Fairness scheduling with the minimum transmission data rate taken into account is proposed by, for example, JP2005-130053A. In JP2005-130053A, it is proposed that Cn=Rn/(avrgRn−targetR) is to be used instead of Cn=Rn/avrgRn as an evaluation function Cn. In the equation, targetR indicates the minimum transmission data rate.
In the 3GPP, the specification of the Evolved UTRA and UTRAN (also referred to as Long Term Evolution or Super 3G) as a further high-speed packet communication system is defined. However, the Evolved UTRA and UTRAN (also referred to as Long Term Evolution or Super 3G) has almost the same characteristics, that is, using a shared channel, performing the AMC, performing scheduling about the allocation of the shared channel, etc., as the above-mentioned HSDPA.
In JP2002-232941A, the amount of uplink interference and the total downlink transmission power when a call is admitted are estimated, and it is determined whether or not the estimated amount of uplink interference is equal to or higher than a threshold value of an amount of interference, whether or not the total downlink transmission power is equal to or higher than a transmission power threshold value, and whether or not the remaining and unused expansion code resources are equal to or lower than an expansion code threshold value. Depending on the determination result, it is controlled whether or not a call admission request is to be recognized.
In JP2002-223239A, depending on the type of service or the priority, the admission of a new call is controlled.
JP2002-217956A controls the admission of a new call depending on the resource use status and the number of packet users.
As described above, there is a method of not admitting a call of a new mobile station when a total number of mobile stations performing communications in a corresponding cell exceeds a predetermined threshold value as a call admission control method in a communication system which transmits a packet to a plurality of mobile stations.
However, the above-mentioned conventional call admission control method has the disadvantage of not able to appropriately controlling admission of a call depending on the method of generating packet data and the mode of a cell. That is, when the conventional call admission control method is used, there occurs the problem that the transmission data rate of a mobile station in a capacity limit depends on the method of generating packet data and the mode of a cell.